Continually operating fluidized bed dryer for drying loose material

ABSTRACT

A continuously operating fluidized bed dryer for loose material such as granulated material comprises a housing in which the fluidized bed is generated by feeding the material into the housing and fluidizing the same by a flow of pressurized air directed upon the material in the housing. Two or more meshing conveyor screws are rotatably mounted in the housing substantially occupying the space therein within which the fluidized bed is generated. The meshing turns or windings of the screws define a plurality of individual fluidizing chambers by which the material fed into the housing at the intake end thereof is conveyed to the discharge end of the housing.

n Unite States Qatent 1151 3,646,689

I Kuchenthal et al. Mar. 7, 1972 [54] CONTINUALLY OPERATING 2,444,3836/1948 FLUIDIZED BED DRYER-FOR DRYING 1;"

LOOSE MATERIAL 3,118,744 1/1964 [72] Inventors: Gunther Kuchenthal,Ludwigsburg-Pflug- 3,425,640 2/1969 felden; Wolfgang .Hentzschel,Stuttgart- 3,452,864 7/1969 7 Sud, th of rm y 3,549,000 12/1970 [73]Assignee: Werner 8: Pfleiderer, Stuttgard-Feuerbach, Germany PrimaryExaminerFreder1ck L. Matteson Assistant Examiner-Robert A. Dua [22] led:Sept 1970 Attorney-Plane, Baxley, & Spiecens [21] Appl. No.: 71,547 Y[57] ABSTRACT 30] Foreign Applimfion Priority mm; A continuouslyoperating fluidized bed dryer for loose material such as granulatedmaterial comprises a housing in which Sept. 17, 1969 Germany ..P 19 46997.7 I the fluidized bed is generated by feeding the material into thehousing andfluidizing the same by a flow of pressurized air [52] CL Idirected upon the material in the housing. Two or more mesh 207/1 ingconveyor screws are rotatably mountedin the housing sub- [51] In. F26,17/20 stantially occupying the space therein within which the 58] FieldofSearch...............:547l68 174 183 236 571) fluidized bed isgenerated- The meshingmmsmwindingwl 34/57 B 10, 182; 198/213; 362 29,50;23 the screws define a plurality of individual fluidizing chambers 7284; 209/11, 464 by which the material fed into the housing at theintake end thereof is conveyed to the discharge end of the housing. [56]References C ted 8 Claims 2 Drawing Figures UNITED STATES PATENTSCONTINUALLY OPERATING FLUIDIZED BED DRYER FOR DRYING LOOSE MATERIAL Theinvention relates to a continually operating fluidized bed dryer fordrying loose material such as granulated or pulverized material, andmore particularly to a fluidized bed dryer of this kind includingconveying means in a housing within which the fluidized bed isgenerated.

BACKGROUND With known continually operating fluidized bed dryers of thegeneral kind above referred to it is not possible, or at least not to asatisfactory extent, to maintain a selected definite dwell time for thematerial to be dried by the fluidized bed generated in the dryer. Dryersas now conventionally used provide only a limited forcible movement ofthe material and have thus a more or less broad range of the dwell timefor the material to be dried. There are known, for instance, devicesequipped with a single conveyor screw located at or near the bottom partof the fluidized bed in the dryer housing.

It has also been found that differently sized particles and variationsin the number of particles of a specific size present within a givenunit of the material as unavoidably occur in the material, result in theformation of layers or strata in the material which forms the fluidizedbed. As a result, conveyance of the strata of material toward thedischarge end of the housing occurs at different velocities. Tests haveshown that uniform conveyance of stratified material cannot be reliablyobtained by a single conveyor screw or similar simple conveyors mountedat or near the bottom of the fluidized bed as generated within thehousing of the dryer. Such nonuniform conveyance of the material is dueto the free spaces which are left between the conveying means such asthe windings of a conveyor screw and outside the screw windings or otherconveying means. These free spaces permit the particles to jump part ofthe conveying means since the movements of particles within the bed arecompletely at random.

Due to such uncontrollable or only poorly controllable length of thedwell time of the material to be dried, over-heating of sensitivematerial to be dried can and does occur. Moreover, the dwell time cannotbe accurately selected in accordance with the drying requirements of aspecific material. As a result, additional drying of the material afterit has been discharged from the dryer is often still necessary.

THE INVENTION It is an object of the invention to provide a novel andimproved fluidized bed dryer of the general kind above referred to inwhich the dwell time of the loose material to be dried can be accuratelycontrolled. As a result, a dryer according to the invention has theadvantage that the material when discharged from the dryer is fullydried without danger of deterioration of the material by overheating.

Another important object of the invention is to provide a novel andimproved fluidized bed dryer of the general kind above referred to inwhich stratification of the material during the fluidizing operation iseffectively avoided thereby assuring that the material is homogeneouswhen discharged.

SUMMARY OF THE INVENTION The aforepointed out objects, features andadvantages, and other objects, features and advantages which will bepointed out hereinafier are obtained by providing in the housing of afluidized bed dryer of the kind above referred to, conveying meanscomprising two or more meshing screw conveyors. These conveyor screwsare disposed within the space in which the fluidized bed is generated.The meshing turns or windings of the conveyor screws form a plurality oflengthwise aligned fluidizing chambers. The screws have advantageouslyequal outer diameters and are driven at adjustable rotational speeds.

The dimensions of the screws are preferably so selected that they extendthrough substantially the entire fluidized bed, the dimensions of which,in turn, are defined by the dimensions and the shape of the housing. Itis particularly advantageous to arrange the conveyor screws so that theysatisfy the equation: H=(nl )-0+D/2, where H maximal height of the bedlayer within the housing;

n= number of conveyor screws;

a= distance of the rotational axes of the screws;

D= the outer diameter of the screws.

In addition to the aforepointed out advantages, a fluidized bed dryeraccording to the invention offers other important advantages. As is wellknown, a thick fluidized bed is desirable. Good stabilizatidn of afluidized bed requires a minimum volume of air in relation to the lossof pressure at the housing bottom through which the pressure air flow issupplied to the housing. If the fluidized bed is comparatively shallow,the pressurized air flow through the bottom of the housing isinsufficiently utilized for the actual fluidizing or whirling action.Accordingly, a highly efficient operation of the dryer can be onlyobtained when the bed has a considerable height or thickness. Inaddition, better utilization of the airflow with a high or thick bedalso results in an increased passage of material through the dryer foran equal dwell time of the material per unit of time.

The use of meshing screw conveyors arranged in accordance with theinvention also makes unnecessary the use of specific devices forbreaking up lumps, etc., which tend to interfere with the fluidizingaction. As is evident, conveyor screws according to the invention willautomatically disintegrate any lumps which may be present in thematerial as it is fed into the dryer.

According to another advantageous aspect of the invention the drivemeans for the screws are so arranged that the rotational velocity of thescrews can be gradually varied. As it is evident, such gradualvariations of the rotational speed of the screws permits accurateadjustment of the dwell time in accordance with the requirements of aspecific operation.

The invention also provides that the windings of the conveyor screws areformed by helically wound strips such as metal strips. Such strips arewound about a rather thin core shaft suitably secured thereto. The useof screws of this kind reduces the total space occupied by the screwswithin the fluidized bed, thus correspondingly increasing the spaceavailable for the material to be dried without in any way interferingwith the fluidizing action.

The fluidized bed according to the invention may be visualized as beingformed by a row of individual fluidizing chambers, or as the sum totalof several fluidizing actions carried out in the individual fluidizingchambers. Such chambers are formed by the windings of the meshingconveyor screws, as previously described. The rotation of the chamberscauses conveyance of the material to be dried toward the discharge endof the housing at a speed controlled by the selected rotational speed ofthe screws. Tests have shown that when the winding pitch A=0.6D is used,good results are obtained. It has been further found that it isadvantageous to arrange the meshing screws so that the flanks of themeshing windings are as close one to another as practical and that theapices of the screw winding extend as closely as possible to the coreshaft of the respective other screw. In other words, it is advantageousto operate the screw with tight mutual engagement.

DETAILED DESCRIPTION OF THE DRAWING In the accompanying drawing, apreferred embodiment of the invention is shown by way of illustrationand not by way of limitation. In the drawing:

FIG. 1 is an elevational diagrammatic view, partly in section, of afluidized bed dryer according to the invention; and

FIG. 2 is a fragmentary section takenon line lI-II of FIG. 1.

In the figures, only those components of the dryer are shown as areessential for the understanding of the invention.

Referring now to the figures more in detail, the exemplified fluidizedbed dryer comprises two conveyor screws 1. Various types of conventionalconveyor screws may be used but it is particularly advantageous toprovide screws which as shown have a comparatively thin core shaft 2about which a strip 20 is spirally or helically wound. The strip may bevisualized as being suitably secured to thecore shaft as by welding.

As it is clearly shown, the two screws are in tight mesh with each otherand the windings thereof define a plurality of chambers 3 along thelengths of the screws. The individual aligned chambers assure that theentire fluidized bed generated in a housing 15 of the dryer is dividedin a plurality of individual fluidizing actions. As is also evident,rotation of the screws will convey material within the housing from anintake or feed end B toward a discharge C.

While only two meshing screws are shown, it should be understood that itis within the concept of the invention to provide three or moresuperimposed meshing screws.

As it is shown in FIG. 2, the aforereferred to distance a between therotational axes of the two screws is designated by 4. FIG. 2 alsoindicates the height or thickness H of the bed within the dryer.

The material to be dried is preferably fed into the dryer via a bucketwheel gate 5 and discharged via a bucket wheel gate 6. Other suitableagitating devices may, of course, be provided at the inlet end and thedischarge end of the dryer housing.

Pressurized air is supplied to the dryer via a duct D. The air, afterbeing preferably heated in a suitable conventional heater 7, flows intoa distributing chamber 8 and from this chamber into the space of thedryer housing in which the fluidized bed is to be formed, via anapertured bottom wall 9. This bottom wall may be a sieve or it may be asintered wall; it is only essential that it is sufliciently airpervious.

Both screws are driven by a motor 10, the output speed of which shouldbe visualized as being gradually variable. Variable speed gears or othersuitable speed-regulating means are well known and readily available forthe purpose; they are not described in detail as they do not constitutepart of the invention. As pointed out before, a gradual variation of therotational speed of the screws permits precise adjustment of the dwelltime of the material within the fluidized bed, as the speed of thescrews controls the speed with which the material is conveyed and thusthe dwell time.

A stilling or calming zone 11 for the pressurized air is provided withinthe housing above the screws and thus the bed proper. Fine particles asmay be carried along by the spent air, that is, by the air flow abovethe screws are separated from the airflow, for instance by means of aconventional cyclone separator 12 which is connected, preferablytangentially, to chamber 11 by a duct lla. As it is shown andconventional for such separators, the purified air is discharged fromthe separator through a duct E while the collected particles (thesocalled heavies in this case) leave the separator through a duct F. Abucket wheel-type gate 12a or other suitable agitators may be includedin the discharge duct F for the heavies as they leave the separator.

Bottom wall 9 through which the airflow generating the fluidized bedwithin housing passes has, as previously stated, a multitude ofapertures. It has been found to be particularly advantageous to arrangethese apertures in the form of nozzles slanted toward the discharge endC of the housing. Such orientation of the nozzles imparts to particlesof the material and particularly to coarse particles, a movement towardthe discharge end; in other words, the slanted airflow through bottom 9supplements the conveying force provided by screws 1. The generation ofsuch auxiliary conveying force is of particular importance because spaceportions 13 directly adjacent to bottom 9 are not within the range ofthe direct conveying action of the screws, as it is clearly shown in FIG. 2. Hence, the conveying force applied to material in spaces 13 tendsto be at a minimum.

Theoretically, it would be possible to eliminate the more or less deadspaces 13 by shaping bottom 9 substantially in confonnity with theperipheral outline of lower conveyor screw 1. As stated before, it isdesirable to dimension and arrange the conveyor screws so that theyextend substantially through the entire space in which the fluidized bedis maintained in housing 15 during operation. However, in actualpractice a housing bottom-shaped, that is curved, in conformity with theperipheral outline of the respective conveyor would adversely 5 affectthe stability of the fluidized bed to such an extent that the advantageobtained by eliminating spaces 13 is more than nullified.

As it is evident and within the concept of the invention, two or moredryers can be arranged side-by-side and a common stilling or calmingzone 11 for all the dryers may be provided in the housing.

While the invention has been described in detail with respect to acertain now preferred example and embodiment of the invention, it willbe understood by those skilled in the art, after understanding theinvention, that various changes and modifications may be made withoutdeparting from the spirit and scope of the invention, and it isintended, therefore, to cover all such changes and modifications in theappended claims.

What is claimed is:

1. A continuously operating fluidized bed dryer for drying loosematerial comprising in combination:

a closed housing for generating therein a fluidized bed;

two or more meshing conveyor screws of equal diameter rotatably mountedin the housing in superimposed relationship, said screws having anintake and a discharge end and the meshing windings of said screwsdefining a plurality of individual fluidizing chambers along the lengthsof the screws, said housing including an inlet adjacent to the intakeend of the screws for feeding loose material to be dried to the intakeend of the screws and an outlet adjacent to the discharge end of thescrews for discharging dried material from the housing, said chambersconveying the material from the inlet of the housing to the outletthereof and fluidizing the material while conveying the same; and aconduit communicating with the housing below said conveyor screws forfeeding pressurized air into the housing to generate therein a layer ofwhirling loose material constituting a fluidizing bed,

said conveyor screws being disposed and dimensioned to extend throughsubstantially the entire space occupied by said bed within the housingand to satisfy the equation:

khr-l )rH-D/Z wherein H is the maximal height of the fluidized bed;

n is the number of conveyor screws;

a is the distance between the rotational axes of the screws;

and

D is the outer diameter of the screws.

2. The dryer according to claim 1 and comprising a second conduitcommunicating with space within the housing above said conveyor screwsfor discharging spent air from the housing and particles of materialescaping from said chambers and entrained in the air.

3. The dryer according to claim 2 wherein said housing includes abovethe conveyor screws an air-stilling space, said second conduitcommunicating with said air-stilling space.

4. The dryer according to claim 2 wherein said second conduit includes acyclone separator for liberating the spend air from particles entrainedtherein.

5. The dryer according to claim 1 wherein said conduit communicates withthe housing through apertures in the bottom wall of the housing.

6. The dryer according to claim 1 wherein a drive means with graduallyvariable speed is drivingly coupled to said conveyor screws.

7. The dryer according to claim 1 wherein each of said conveyor screwscomprises a shaft having mounted thereon a helically wound strip, saidstrips defining the windings of the screws.

8. The dryer according to claim 7 wherein the windings of adjacentconveyor screws mesh tightly one with another and have outer diametersextending closely adjacent to the respective shafts.

2. The dryer according to claim 1 and comprising a second conduitcommunicating with space within the housing above said conveyor screwsfor discharging spent air from the housing and particles of materialescaping from said chambers and entrained in the air.
 3. The dryeraccording to claim 2 wherein said housing includes above the conveyorscrews an air-stilling space, said second conduit communicating withsaid air-stilling space.
 4. The dryer according to claim 2 wherein saidsecond conduit includes a cyclone separator for liberating the spend airfrom particles entrained therein.
 5. The dryer according to claim 1wherein said conduit communicates with the housing through apertures inthe bottom wall of the housing.
 6. The dryer according to claim 1wherein a drive means with gradually variable speed is drivingly coupledto said conveyor screws.
 7. The dryer according to claim 1 wherein eachof said conveyor screws comprises a shaft having mounted thereon ahelically wound strip, said strips defining the windings of the screws.8. The dryer according to claim 7 wherein the windings of adjacentconveyor screws mesh tightly one with another and have outer diametersextending closely adjacent to the respective shafts.